The aim of this proposal is to test in primates a new method for producing tolerance to solid organ grafts implantation of donor CD34+ stem cells into the recipient thymus. In pilot studies, we achieved 1) peripheral blood microchimerism in baboons with or without immunosuppression; 2) specific prolongation of donor vs. third-party skin graft survival; and 3) absence of graft vs. host disease at 14 months. We are now testing the validity and feasibility of this approach in a juvenile baboon model as a pre-clinical trial. In the past year, we found that selected subsets of CD34+ cells were less allostimulatory in mixed lymphocyte cultures, and thus might be more promising as tolerogens. These stem cell subfractions produced peripheral blood microchimerism (donor cells circulating in recipient blood) more reliably and more persistently than whole stem cell preparations, also promising for tolerogenic function. Finally, in baboons tested with cardiac grafts after intra th ymic injections of stem cell subfractions, the survival of one of three cardiac grafts was prolonged to more than twice the control graft survival time. Grafts that did not show prolongation of survival were in animals that became sensitized by the stem cell injection. We are now investigating the effect of transient immunosuppression at the time of stem cell implantation in order to obviate the problems of sensitization. FUNDING NIH grants RR00166 and AI37747. Allen, M.D., Weyhrich, J., Akimoto, H., Gaur, L., Hall, J., Dalesandro, J., Sai, S., Thomas, R., Nelson, K.A., and Andrews, R.G. Prolonged allogeneic and xenogeneic microchimerism can be achieved in unmatched primates without immunosuppression by direct intrathymic implantation of CD34+ donor marrow cells. Cell. Immunol. 181:127-138, 1998. Allen, M.D., Gaur, L.K., Nelson, K., de Fries, R., Delio, P., Anasetti, C., and Andrews, R.G. Microchimerism is induced in unrelated primates without immunosuppression by immature CD34+ donor stem cells. Abstr. 10th Intl Congr. Immunol., New Delhi, India, November 1998.